This Program Project is a coordinated multidisciplinary program of clinical and basic research into the biology of marrow transplantation as manifested in patients undergoing marrow transplantation for aplastic anemia and leukemia. The studies in progress are designed to assess the contribution of various genetic, immunologic and hematopoietic functions to engraftment, graft vs. host disease or increased susceptibility to infection. Immunogenetic analyses are designed to distinguish those alloantigenic systems within and outside the major histocompatibility complex which must be common to donor and host if engraftment is to be regularly achieved and severe graft vs. host disease evaded. Studies of hematopoietic function are principally concerned with the biological processes which enhance or limit regeneration of normal hematologic function in the transplant recipient. Similarly, studies of the process of immune reconstitution in the marrow transplant recipient are designed to determine those functions or dysfunctions of the lymphoreticular system of the host (including lymphocytes, phagocytes, and the organs in which they develop) which may limit engraftment and functional reconstitution and, conversely, those of the marrow graft which influence the development and clinical expression of graft vs. host disease or complicating infections. The patients participating in these studies are individuals with aplastic anemia or leukemia who receive transplants of marrow after treatment with standardized experimental courses of radiotherapy and/or chemotherapy for cytoreduction or immunosuppression. The clinical and experimental results of these studies will then be used as a reference base for the development and selection of new therapeutic protocols for randomized trials.